list: minor fixes in ListResource and ValidateListResourceConfig

[stephybun]

Description

• Calls Configure in the ListResource RPC
• Removes a ListResource call in the ValidateListResourceConfig

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

None

[stephybun]

@dbanck in order to capture any warning diagnostics that providers might set during the Configure call on line 115 above there might be cases where we need to append an empty ListResult with only a warning diagnostic in it to the stream.

How would this be handled by Terraform and would this be problematic?

[dbanck]

In the current implementation we stop on errors, but continue on warnings.

But we also raise an error when a result has no identity.

https://github.com/hashicorp/terraform/blob/dec0edfd5afff7169f94424aae0590dddfddee8a/internal/plugin/grpc_provider.go#L1358-L1362

[dbanck]

So appending a result with only a warning will still result in an error in Terraform, because the identity is missing

[austinvalle]

LGTM, we can add some tests later when we have a plan on how we want to handle those warning diagnostics 🚀